Question

When we switch on the fan in a closed room, the temperature of the air molecules:
A) increases.
B) decreases.
C) remains unchanged.
D) may increase or decrease depending on the speed of rotation of the fan.

Answer 1

Hint: This situation is related with a closed system. Here we assume that there is no energy intake or giving to the surroundings. In the closed system transfers and transformations are involved in the mechanical energy only.

Complete step by step answer:
The first law of thermodynamics states that, in any physical or chemical process, the energy of the system and surroundings taken together remains constant. It can also be stated as, energy can neither be created nor be destroyed but can only be converted from one form to another. This law can be applied in this situation also.
A fan converts its potential energy to kinetic energy using the electrical energy supplied to it. We know that air molecules are found everywhere. When this fan starts rotating, the air molecules present in the room also start rotating, that is, their potential energy gets transformed to kinetic energy. As there is no other way to transform this energy to any other form, the air molecules in rotation transform their kinetic energy to thermal or heat energy. In total, the electrical energy supplied to the fan motor converts the fan’s potential energy to kinetic energy which in turn converts the potential energy of air molecules to kinetic energy and then into thermal energy. Thus, we can see that the temperature of the air molecules will increase when we switch on the fan in a closed room. This is a good example for the first law of thermodynamics.

So, the correct answer is Option A.

Additional information:
When heat is added to a gas, its molecules move faster in all the directions at random. Thus, heat can be regarded as a mode which stimulates the random motion.

Note: Heat refers to random form of energy. Work refers to organised forms of energy. In a closed system mass inside the system is constant throughout the process because there is no energy exchange is possible in this system and we can control the flow of energy.